The Accurate Reloading Forums
Barrel Lenght and Presure
05 July 2004, 03:46
steve4102Barrel Lenght and Presure
If a short barrel produces less velocity with the same load as a long barrel, then is the pressure also less in the shorter barrel? I guess my question is, does barrel length have an effect on pressure as well as velocity, and are they related?
Peak pressure normally occurs before the bullet has traveled very far so you would have to drastically shorten a rifle barrel to reduce it.
05 July 2004, 05:04
Cold BoreSteve-
Peak pressure should be the same, a longer barrel just gives the expanding powder gasses more time to "push" the bullet. (Once it leaves the muzzle it starts slowing down, obviously, so the longer barrel gives it a better start due to "time" of pushing, rather than higher "pressure").
05 July 2004, 05:10
TailgunnerThe longer barrel gives the gas a longer TIME to accelerate the bullet. Peak chamber pressure is still the same however, as is the pressure at any given point along the barrel.
Crude analogy would be to accelerate your car as fast as possiable for 1/10 of a mile and look at the speedometer, do it again but go 2/10 and look again. The acceleration rate for the first 100' (peak pressure) is the same for both, isn't it?.
The converse of this is that if you hold a short barrel velocity constant, and work up a long barrel load to equal that velocity with the same or slower powder, the pressure will drop.
05 July 2004, 11:14
steve4102OK, next question, I have a Browning Bar 300WSM with BOSS. The barrel is 23" with BOSS and 20" without. Would this be considered a 20" barrel? If so, then would the BOSS CR (Conventional Recoil- No muzzle brake) still be 20" or would the fact that it has no muzzle brake bring it back to 23"? I am asking this because my load development is giving me velocities much lower than listed. Browning is way behind on the delivery of the BOSS CR so I can't compare the two.
05 July 2004, 12:46
4bambamThe short answer is, both short and long barrel guns will see the same peak pressure, barrel lenght is irrelevant to maximum pressure. Smokeless powder is fuel. It will only generate energy until it is consumed, and the smaller the space for burning, the greater the powder's ability to pressurize the area.
It functions as a 20 inch bbl, since the pressure drops dramatically when you enter the brake.
05 July 2004, 14:07
stubblejumperQuote:
I am asking this because my load development is giving me velocities much lower than listed.
That is normal for a boss equipped rifle.One of the disadvantages of the boss equipped rifles is that the shorter barrel produces less velocity reguardless of whether the standard boss or the cr is used..
06 July 2004, 00:21
<eldeguello>Quote:
If a short barrel produces less velocity with the same load as a long barrel, then is the pressure also less in the shorter barrel? I guess my question is, does barrel length have an effect on pressure as well as velocity, and are they related?
NO! The reason velocity is less in a shorter barrel is because the pressure has less time to accelerate the bullet. There is NO relationship between barrel length and peak pressure! 
As far as effective barrel length is concerned, as soon as it is possible for the gasses to completely blow past or out from behind the bullet, they essentially stop accelerating it! However, I have noted that the velocities given in the manuals these days seem to be higher than what I get when I actually chronograph them, even in published barrel lengths. The older manuals seem to be closer to actual performance levels than some of the new ones. This is particularly true of the recent Nosler data! 
06 July 2004, 04:45
Ol` JoeQuote:
The older manuals seem to be closer to actual performance levels than some of the new ones. This is particularly true of the recent Nosler data!
I`ve been led to believe the manuals lately are useing pressure barrels made to saami spec`s in place of factory rifles. The test barrels are much more uniform in the bore and tighter than the one on your old Remchester. The bullets not incountering alternate loose, tight, and rough conditions in its travel helps it gain a few fps.
The barrel lgt has no effect on pressure but does effect the time under pressure of the bullet. If you can find a graph showing the pressure curve of a cartridge you`ll note that although it drops it still shows pressures in tons/per/inch at the muzzel even in 30" or longer barrels. Depending on cartridge. The bullet may not be excelerating at the same rate at 24" as 15" but it`s still being pushed and gaining vel.
06 July 2004, 14:26
4bambamYou are wrong, there is a PEAK PRESSURE in both short and long barrel lenghts. You take the 44 mag for example in a pistol. And you say you have about 38,000 PSI when the bullet has traveled 4/10 ths of an inch. At 1 inch of travel, pressure has dropped to 32,000PSI, at 2"-22,000 PSI ect. Muzzle pressure with a shorter barrel, the pressure at the crown is higher with a short barrel; 14,000-15,000, half as much for a 7.5" barrel and one of the primary reasons short barreled guns have unpleasant muzzle blast and muzzle flip.
There are 3 signicant pressure levels at work when a cartridge is fired; Maximum, Average and Muzzle
07 July 2004, 04:33
RicochetSo why are you saying eldeguello's wrong? He said: "There is NO relationship between barrel length and peak pressure!" At any practical barrel length, that's perfectly true. I don't think we're talking about cutting our rifles down to snubbies with 1" of bullet travel to the muzzle here.
07 July 2004, 13:54
4bambamLike I said, both short and long barrel guns will see the same Peak pressure
Than I mentioned the 3 levels of pressure. As soon as you ignite the primer and the powder charge, the bullet begins to move forward, as the bullet moves forward, it leaves an increasing volume behind it for burning powder to expand. As the increase in volume grows at a rate faster than gases expanding from burning powder, pressure in the chamber and bore diminish.
That 1 inch or so of barrel was an example
07 July 2004, 15:42
RicochetI understood all that. And it in no way disagrees with what eldeguello said. He only said that the peak pressure was unaffected by the barrel length. That's why I asked.
07 July 2004, 15:54
DigitalDanI always thought that revolvers achieved peak pressure before the bullet base left the cylinder and for most it drops like a lead anvil after that. Maybe the Casull as an exception due to small gap dimension?
True/False? Anybody seen pressure curves from a SB/RH or Smith .44 Mag?
Never cared that much for them anyway.